Feeding of powders

ABSTRACT

CONSTANT MASS-RATE FEEDING OF POWDERS IS IMPROVED BY SUPPLYING THE POWDER TO A FEEDER IN FLUID FORM, AND AT A UNIFORM BULK DENSITY, FROM A BED OF THE POWDER, THE DEPTH OF THE POWDER BED IS MAINTAINED ESSENTIALLY CONSTANT, THUS PROVIDING A MORE CONSTANT HEAD OF PRESSURE UPON THE POWDER BEING SUPPLIED TO THE FEEDER.

Dec. 14, 1971 R. E. DRISCOLL 3,627,555

FEEDING OF POWDERS Filed Sept. 10, 1968 INVENTOR RICHARD E. DRISCOLL BY; p

ATTORNEY United States Patent 3,627,555 FEEDING OF POWDERS Richard E. Driscoll, Monroe, La., assignor to Columbian Carbon Company, New York, N. Filed Sept. 10, 1968, Ser. No. 758,880 Int. Cl. 01% 1/48 US. Cl. 106-307 5 Claims ABSTRACT OF THE DISCLOSURE Constant mass-rate feeding of powders is improved by supplying the powder to a feeder in fluid form, and at a uniform bulk density, from a bed of the powder. The depth of the powder bed is maintained essentially constant, thus providing a more constant head of pressure upon the powder being supplied to the feeder.

BACKGROUND OF THE INVENTION The term powder as used herein refers to essentially dry and stable particulate solids which are naturally fluid or which can be rendered fluid by aeration with a gas which is nonreactive with the powder. The term fluid or fluidized as used herein refers to powders which are aerated to the extent that they behave almost as a liquid, e.g. they pour freely without bridging and plugging, conform readily to the outlines of a container, and will of their own accord form into a static bed which is essentially level at the top.

In the processing of powders, it is frequently necessary to provide a stream of the powder which flows at an essentially constant mass-rate. When it is desirable, for instance, to continuously combine the powder with other materials at fixed proportions, it is often times necessary to feed the powder in dry form, and accurate proportioning of the resultant mixture cannot be accomplished unless feeding of the powder to the combining zone is maintained at a constant rate. While suitably accurate constant rate feeding of gases, liquids or dense particulate solids can generally be accomplished without too much difliculty, accurate mass-rate feeding of dry powders has been a long standing problem.

It has been common practice to supply powder to volumetric displacement or weigh-type feeders from a standing bed of the powder contained within a suitable receptacle. The depth of the bed will fluctuate considerably if the rates at which powder is added to and removed from the bed are significantly unequal. If the powder flows freely and readily conforms to the outline of its container, the bed will have a head pressure which depends upon the depth of the bed, and which will vary proportionately with changes in the bed depth. It has now been discovered that variations in the static head pressure of the powder being supplied to the feeder can cause erratic mass-rate delivery of the powder by the feederwhether the feeder be a volumetric displacement or weigh type.

The constant rate feeding of fluid powder is also adversely affected if the bulk density of the powder intro duced into the feeder is significantly nonuniform. Obviously, mass-rate delivery of powder from a volumetric displacement feeder will fluctuate if the bulk density of the powder supplied thereto is nonuniform, and it has also been observed that delivery from a weight-type feeder may also fluctuate unless the powder is supplied to it at a substantially uniform bulk density, particularly when the powder is in the low bulk density range of about 3 to lbs. per cubic foot.

Problems are also experienced with the feeding of powders which lack or lose fluidity in the powder bed, since the powder may pack or bridge to the extent that free flow to the feeder is interrupted. However, it should be 3,627,555 Patented Dec. 14, 1971 pointed out that highly fluid powders, e.g. powders having a bulk density below about 15 pounds per cubic feet, are particularly diflicult to feed at a constant mass-rate, since they are more subject to variation in bulk density and are more difficult to meter accurately, even when the bulk density of the powder remains essentially uniform, unless the powder is supplied to the feeder at a fixed head pressure. Thus, the constant rate feeding of essentially dry powders can be improved by supplying the powder to the feeder in a fluid form having an essentially uniform bulk density, and under an essentially constant head pressure.

It is therefore an object of this invention to improve the feeding of a powder whereby the powder may be fed at a more constant mass-rate.

Another object of this invention is to maintain an essentially constant depth in a bed of fluid powder supplying a powder feeder adapted to deliver the powder at an essentially constant mass-rate.

Still another object of this invention is to supply powder to a powder feeder at a more constant head pressure.

Yet another object is to supply fluid powder to a powder feeder at a more uniform bulk density.

Even another object of the invention is to deliver essentially dry, fluid powder into a combining zone at a constant mass-rate for accurately proportioned combination of the powder with another material.

Other objects and advantages of the invention will become apparent from the following description and the appended claims.

In accordance with the present invention, a bed of fluid powder is formed in a holding receptacle from powder which is continuously supplied to the interior of the receptacle, and the fluid powder is continuously fed from the bed into a suitable feeder through a first powder removal point, the powder removal rate from the bed through this point being less on the average than the rate at which powder is continuously supplied to the bed. However, an essentially constant bed depth is maintained by discharging fluid powder through a second removal point located at a higher elevation in the bed than the first said point. Since the average rate at which powder is supplied to the bed is greater than the average rate at which powder is discharged through the first removal point, the bed depth would continuously increase if it were not for the fact that the powder is also removed from the bed through the second removal point located at a higher elevation. In practice, powder is withdrawn from the bed through the second removal point at a rate sufficient to prevent continuous increase of the bed height, so that the depth is maintained and established at a height fixed by the elevation level of the second removal point. More specifically, powder is removed through the second powder removal point at a rate which essentially equals the diiference between the total rate at which powder is supplied to the bed and the rate at which powder is discharged through the first removal point. Since the head pressure exerted by the bed at the first removal point varies with the depth of the bed, a more constant head pressure is thus established by maintaining the bed of powder at a fixed depth.

Many low bulk density powders are highly fluid in the natural state and will remain fluid unless subjected to mechanical compression, agitation, or prolonged settling which results in substantial deaeration. Thus, it is not essential to the invention, in its broadest aspects, that the powder in the bed be subjected to continuous aeration with a nonreactive gas to maintain the powder in a fluid condition and at a uniform bulk density. Since, however, the powder of the bed may in some instances be relatively dense, or otherwise subject to packing or bridging due to high head pressure or a tendency to settle rapidly, a process of aeration or reaeration may be employed to maintain the powder fluid and/or to establish uniform density of the powder at the first powder removal point. A nonreactive gas can, for instance, be introduced into the holding receptacle in an amount which renders the powder fluid and maintains a uniform density in the region of the bed just ahead of the first powder removal point. Alternatively, powder may be removed from the receptacle from a locus near the bottom of the bed, agitated in the presence of a nonreactive gas to effect reaeration, and returned to the top of the bed in a fluid condition. If the powder is thus reaerated and recirculated from the bottom to the top of the bed at a rate which approaches or exceeds the natural settling rate of the powder in the bed, fluidity and substantially uniform bulk density can be maintained throughout the bed.

The present invention can be employed to facilitate accurate metering of the powder when it is desirable to treat the powder at fixed mass-rate proportions with a reactive or nonreactive gas or liquid, or when mixing the powder with another solid material.

Thus, the invention can be employed for supplying powder at a constant mass-rate in the following continuous processes:

(A) Treatment of the powder particles with a reactive gas or vapor whereby the surface chemical properties of the particles are altered, such as in the treatment of carbon black with oxidizing gases or the treatment of finelydivided silica with steam.

(B) Mixture of the powder particles with a nonreactive gas, such as in the formation of aerosols which may be employed, for instance, for conveying the powder pneumatically.

((1) Treatment of the powder particles with a reactive liquid whereby the surface chemical properties of the particles are altered, such as in the treatment of carbon black with oxidizing acids or the treatment of finelydivided silica with amines.

(-D) Dissolution or extraction of the powder particles in a liquid solvent, such as the dissolving of finely-divided silica in hydrofluoric acid or the extraction of powdered coal with liquid hydrocarbons to remove a soluble fraction therefrom.

(-E) Treatment of the powder particles with a liquid which is essentially nonreactive with the powder particles, and in which the particles are substantially insoluble, such as in the liquid coating of particles, or the aggregation thereof to form pellets, or in the formation of slurries or pastes of the powder.

(F) Blending of a powder with other solids, such as in the compounding of rubber with powdered reinforcing agents or fillers.

The invention may also be employed to advantage in a process for modifying the properties of carbon black by attrition of the particles as described in U.S. Pat. 3,333,979. As disclosed therein, a stream of carbon black is continuously circulated at a relatively constant massrate through an attrition mill. While a feed stream of unmodified carbon black is charged to the mill, a portion of the recirculated stream is withdrawn from the system as treated product. As is illustrated in FIG. 2 of the patent, the degree to which carbon black structure is reduced by the treatment is considerably dependent on the recycle ratio, i.e. the proportion of recirculated black to unmodified black being fed to the attritioning mill. When it is desirable, for instance, to continuously produce a carbon black having its structure reduced to a specific level, it is most important that the proportioning of unmodified feed to recirculated black be maintained essentially constant lest the quality of the product be nonuniform. Since the unmodified black can be fed to the system in the form of relatively dense pellets, i.e. -25 lbs. per cubic foot, constant rate feeding of that stream is not too difiicult since the pellets flow freely, and the problems of feeding powder which result from variations in head pressure and density are not nearly so serions with pellets. On the other hand, the carbon black which is discharged from the attrition mill is a low bulk density fluid powder, i.e. 3 to 7 lbs per cubic foot, a sizable portion of which must be recirculated to the mill at a constant mass-rate. By employing the present invention, the recirculation rate to the mill may be more precisely controlled by maintaining the bed of black in the surge tank at a constant depth, thus passing black from the first removal point to the feeder which controls the recycle rate, and removing black from the second removal point as the product stream. Accordingly, modification of the properties of the carbon black by this attritioning process may be more precisely controlled by utilizing the present invention in controlling the recirculation rate. It has also been found that use of a feeder for removing a modified-black product stream is not required for maintaining a fixed inventory of black in the system, i.e. the product stream can be obtained by merely allowing the fluid black to overflow from the bed through the second powder removal point.

BRIEF DESCRIPTION OF DRAWINGS FIG. 1 is a somewhat diagrammatical illustration of apparatus arrangements which may be employed in carrying out the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In reference to FIG. 1, an aerosol of powder particles is fed continuously by way of line 1 into a cyclone separator 2. The aerosol is fractionated within the cyclone 2, so that an aerated, -fiuid powder is continuously discharged into the receptacle 3 while the depleted gaseous fraction of the aerosol is discharged from the cyclone through line 4. A bed 5 of the fluid powder is thus created within the receptacle 3. Formation of an aerosol for feeding powder to the bed is not essential, however, since if the powder is already available as a fluid, bulk powder it may be added to the bed directly.

In one embodiment of the invention, fluid powder is continuously discharged from the powder bed '5 through an opening 7 in the wall of receptacle 3 and is passed into a feeder, represented at 8, through a conduit 9, which interconnects the feeder with the opening 7. In this instance, the opening 7 is the first removal point through which powder is discharged from the bed. The metered stream of powder which is delivered by the feeder 8 is passed by way of line 10 to any receiving point where constant rate delivery of the powder is desired.

As previously indicated, it is an object of the invention to provide a fixed depth in the powder bed 5 in order to maintain a constant head pressure on the powder feeder 8. Powder is continuously discharged to the feeder from the bed through a first removal point 7 while powder is continuously added to the bed from cyclone 2 at a rate which, on the average, exceeds the rate at which powder is passed to the feeder. In order to maintain a constant depth in the powder bed 5, it is therefore necessary to remove any excess of powder which might accumulate in the receptacle due to addition of powder to the bed at a faster rate than it is permanently removed therefrom. This is accomplished by removing the excess powder from the bed through conduit 6, which extends vertically into the receptacle, and has an opening 6a in the upper terminus thereof.

It will be noted that opening 6a, which in effect is the second point through powder is removed from the bed, is located at an elevation significantly higher than the opening 7. Consequently, a positive and essentially constant head of pressure is maintained on the feeder 8 by establishing and maintaining a fixed depth of powder in the bed 5 by removing powder from the bed through the second discharge point at a rate such that the sum of the average rates at which powder is removed from the bed through the first and second removal points essentially equals the rate at which powder is supplied to the bed It will be appreciated that with an arrangement as shown in FIG. 1, fluid powder may be discharged through the first and second powder removal points by gravitational flow; i.e. no positive mechanical displacement or suction pressure is required. Powder which is removed through conduit 6 may be passed by way of line 1-1 to a storage point for subsequent reuse, i.e. it may, for instance, be passed to a surge tank and later reformed into the aerosol which is passed by way of line 1 into the cyclone 2.

In the event that the powder has a tendency to settle rapidly or pack or bridge within the receptacle 3, it is desirable to preserve a condition of fluidity in the powder bed, especially at the first powder removal point, and this may be accomplished by introducing a nonreactive gas into the powder bed at a locus near the first removal point, the gas being introduced at a rate which aerates the powder and renders it uniformly fluid. Thus, a fluidizing gas may be introduced into the bed through line 12 and subsequently discharged from receptacle 3 through a vent, represented at 13.

Where preferred, the powder of the bed may be maintained fluid and relatively uniform throughout by a process of reaeration and recirculation. For instance, powder may be removed from the bed 5 through conduit 14 and passed by way of a feeder 15 into an agitation zone, represented at 16, through line 14a. A nonreactive gas may be introduced into the agitation zone 16 through line 17 for formation of an aerosol of the powder which is passed by line 18 into cyclone 19. The aerosol may be formed, for instance, by feeding the powder and the nonreactive gas into a fan or blower within which the mixture is vigorously agitated and propelled through line 18 to the cyclone.

The aerosol is fractionated in cyclone 19 and the reaerated, fluid powder returned to the bed 5. Nonereactive gas which is fractionated from the aerosol may be vented or else recycled by way of line 20 to the agitation zone. In practice, the powder is reaerated and recirculated at a rate which equals or exceeds the settline rate of the powder in the bed, and problems of packing, bridging, and nonuniform density are overcome by maintaining the powder highly fluid throughout the bed at all times. Thus, constant mass-rate delivery of powder by the feeder 8 is improved by supplying powder thereto which is uniform in density and under a constant head pressure, since reaeration and circulation of the powder in the bed is carried on in conjunction with maintaining the powder at a fixed depth.

When employing the invention in an attritioning process as described in US. Pat. 3,333,979, the agitation done 16 can be the grinding chamber of an attrition mill into which unmodified feed carbon black is introduced at a constant mass-rate through line 21 while powder is continuously fed from the bed 5 into the ball mill at a constant mass-rate by means of feeder 15. The rate at which carbon black powder is fed from the bed into the mill is at least as great as the rate at which the unmodified black is fed to the mill, and more preferably two to forty times as great. Accordingly, carbon black from thhe bed 5 is constantly recirculated through the attrition mill while unmodified feed back is added to the system through line 21. Treated product is removed through conduit 6. In such an operation, the feeder 8 need not be employed, in which case the upper opening of conduit 14 becomes the first powder removal point. Furthermore, since carbon black is added to the system through line 21, powder need not be fed to the bed through line 1. After passing through the mill the carbon black is circulated back to the bed in fluid form, either as bulk powder or as an aerosol, as previously described. An essentially constant depth of powder ise maintained in bed 5 by continuous overflow of fluid carbon black powder through opening 6a, the average discharge rate from that point being substantially equal to the rate at which carbon black feed is supplied to the attrition mill. It will be appreciated that all carbon black which passes from the attrition mill is recirculated to the powder bed and supplied to it at a total rate which is greater than that at which powder is removed through the first removal point, thus resulting in product overflow through conduit 6 and a constant depth of powder in the bed 5. The unmodified black may be fed to the attrition mill in powder form, but pellets of the powder can be employed to particular advantage, since they will be disintegrated in the attritioning chamber and the resultant particles subjected to physical modification by attritioning in the chamber. Where desired, a reactive gas may be introduced into the attritioning chamber through line 17 for chemical modification of the carbon black.

It should also be pointed out that the recirculating stream of carbon black can be maintained highly fluid and at a substantially uniform density by repeated passes through the mill, thus aerating the black by agitation in the presence of gases which normally exist within the system, eg air, thus obviating the need for introducing a gas from an external source for the purpose of aeration. The resultant fluid powder can be conveyed to and from the mill in bulk form by means of elevators, screw conveyors, or the like.

While the invention has been described with particular reference to powdered materials in the submicron particle size range, e.g. carbon black, finely-divided silica, etc., and which are naturally flocculent and characterized by low bulk density, it will be understood that the invention is not limited thereto since it can be employed with any essentially dry and stable particulate solid which exist in a fluid state, or which can be rendered fluid, such as expanded polystyrene beads, vermiculite, or the like.

Therefore what is claimed is:

1. In a process for feeding dry powder from a holding recepticle to and through a powder feeder adapted to deliver a stream of the powder at a constant mass-rate, the improvement which comprises the steps of:

(a) forming a bed of fluid powder within said receptacle from powder which is continuously supplied to the receptacle,

(b) continuously discharging powder from said bed into said powder feeder through a first powder removal point, the average rate at which powder is discharged through said first removal point being less than the average rate at which powder is supplied to said bed,

(c) maintaining a constant depth of powder in said bed by discharging excess powder being added thereto through a second powder removal point located at a higher elevation than the first said removal point, the average rate at which powder is removed through the second said point being equal to the difference between the average rate at which powder is supplied to the bed and the average rate at which powder is discharged therefrom through said first removal point,

whereby constant mass-rate feeding of the powder is improved by maintaining a constant head of pressure upon the powder supplied to the feeder.

2. The process of claim 1 including the steps of:

(l) continuously removing powder from the bed from a locus near the bottom thereof,

(2) reaerating the powder removed in step (1) by agitation in the presence of a nonreactive gas, and

(3) returning the reaerated powder of step (2) to the top of the powder bed,

whereby the powder is maintained fluid and at a substantially uniform bulk density throughout the bed.

3. In a process for attritioning carbon black, the improvement which comprises:

(1) supplying unmodified feed carbon black to an attrition mill at a constant mass-rate, the properties of the carbon black particles being modified by attritioning action during passage through said mill,

(2) discharging an attrited stream of carbon black from said mill and continuously supplying it in the form of a fluid powder to a receptacle wherein said carbon black powder is contained as a fluid bed,

black feed stream to said mill is a combination of unmodified carbon black and carbon black which has been recirculated from the said fluid bed, through the mill, and back to the bed.

(3) continuously discharging carbon black powder 5 from said bed into a powder feeder from a first powder removal point, and supplying powder from said bed to said mill at a constant mass-rate through said feeder, attriting said powder and discharging same from said mill in combination with said carbon black of step (2) and thus recirculating it back to said fluid bed of powder, the average rate at which powder is discharged from said first removal point being less References Cited UNITED STATES PATENTS th th t t h d dt 2,905,538 9/1959 McIntire 302-53 X i f gjgf Ia e a W POW er supp 1e 0 2,983,453 5/1961 Bourguetetal 241-26X (4) maintaining a constant depth of powder in said bed 3,333,979 8/1967 Mllhgan 106307 by discharging powder therefrom through a second 3,152,005 10/1964 Tums F X powder removal point located at a higher elevation 3,104,030 9/1963 Howlett 222-56 X than the first said removal point, the average rate at 3,631,921 3/1953 Odell P which powder is removed through the second aid "735,802 2/1956 Jahmg F X point being equal to the difference between the aver- 2,763,596 9/1956 Feldbauer 23'1 F X age rate at which powder is supplied to said fluid bed FO E GN PATENTS and the average rate at which powder is discharged 847,731 9/1960 Great Britain 259 DIG 17 therefrom through said first removal point, and (5) recovering carbon blaclr powder from the second JAMES E. POER Primary Examiner removal point as an attrition-modified carbon black product, while continuously circulating carbon black 3 SNEED Assistant Exammer powder from said fluid bed to said attrition mill through said first powder removal point and from the US mill back to said fluid bed, and whereby the ecarbon 23-1 r; 222--56, 64; 241 1s; 2s9 4, Dig 17 @233? UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent 3,627,555 Dated December 14, 197].

InventoflX) Richard E Dri scoll It is certified that error eppears in the above-identified patent and that said Letters Patent ere hereby corrected as shown below:

DELETE "COLUMBIAN CARBON COMPANY" AS ASSIGNEE,

INSERT CITIES SERVICE COMPANY Signed and sealed this 30th day of May 1972.

(SEAL) Attest:

EDWARD P LFLETGHERJR. I ROBERT GOTTSCHALK Commissioner of Patents Attesting Officer 

